text: The emission intensity density of an atomic transition from the upper state to the lower state is:
  where:
  *  is the density of ions in the upper state,
  *  is the energy of the emitted photon, which is the product of the Planck constant and the transition frequency,
  *  is the Einstein coefficient for the specific transition.
  The population of atomic states N is generally dependent on plasma temperature and density. Generally, the more hot and dense the plasma, the more the higher atomic states are populated. The observance or not-observance of spectral lines from certain ion species can, therefore, help to give a rough estimation of the plasma parameters.
  More accurate results can be obtained by comparing line intensities:
  The transition frequencies and the Einstein coefficients of transitions are well known and listed in various tables as in NIST Atomic Spectra Database. It is often that atomic modeling is required for determination of the population densities  and  as a function of density and temperature. While for the temperature determination of plasma in thermal equilibrium Sahas equation and Boltzmanns formula might be used, the density dependence usually requires atomic modeling.